The present invention relates to an electric steering wheel lock device that is electronically controlled to restrict rotation of a steering wheel.
Vehicles include steering wheel lock devices to restrict rotation of a steering wheel and prevent theft. Japanese Laid-Open Patent Publication No. 2006-335321 (hereafter referred to as the '321 publication) describes an electronically controlled steering wheel lock device. The electronic steering wheel lock device includes a movable lock bar, which is engaged with a steering shaft, a motor, which moves the lock bar, and an electronic control unit, which controls the motor. The electronic control unit switches relays to switch the direction in which current flows through a power supply circuit that supplies current to a motor. The switching of the relays switches the direction in which the motor generates rotation, that is, the movement direction of the lock bar. When the lock bar is in engagement with the steering shaft, rotation of the steering shaft is prohibited. This prohibits rotation of the steering wheel.
In the '321 publication, a field effect transistor (FET) is arranged between the power supply circuit and a power supply (+B). The FET starts and stops the supply of power to the power supply circuit. The FET is controlled by a control unit, which differs from the control unit that switches the power supply circuit. In a state in which the vehicle is being driven (e.g., when a vehicle drive source is actuated), the FET is deactivated and the supply of power to the power supply circuit is stopped. Accordingly, the steering lock of the '321 publication prevents the lock bar from erroneously engaging the steering shaft when the vehicle is being driven.
FIG. 7 illustrates the motor control of the electric steering wheel lock device in the '321 publication. When engaging the lock bar with the steering shaft, a locking relay is activated to close the power supply circuit of the motor. Then, the FET is activated to supply power to the motor.
The relays are mechanical components. Thus, foreign matter, such as dust, may accumulate on a contact of a relay. When the accumulated amount of foreign matter increases, the foreign matter may obstruct conductivity of the relay and stop the supply of current to the motor. For example, as shown in FIG. 7, when deactivating the locking relay to stop supplying power to the motor (time T21), accumulated matter may be entrapped in the relay. In such a case, the accumulated matter obstructs the conductivity of the relay, and the lock bar may remain engaged with the steering shaft.
Normally, to disengage the lock bar from the steering shaft, an unlocking relay is then activated (time T22) and the power supply circuit is closed. Subsequently, the FET is activated to supply power to the motor. However, the conductivity of the locking relay is obstructed by the accumulated matter. Thus, power is not supplied to the motor. As a result, the lock bar may not be disengaged from the steering shaft.